Recently, the need for a large screen image display has been increased mainly for a TV receiver. A rear projector (a rear projection type image display unit) that can realize this with a relatively light weight and a small size has been drawing attention.
The most common rear projector uses red, green and blue monochrome image CRTs as the image sources, enlarges and projects the images on the CRTs with the corresponding three projection lenses, superimposes the images on the screen, and displays the images as a color image. The basic structure of this rear projector is schematically shown in FIG. 8.
In FIG. 8, 1 denotes a CRT, 2 denotes a projection lens, and R, G and B correspond to red, green and blue monochrome images respectively. Three primary color images formed on the CRTs 1 are enlarged and projected by the projection lenses 2, and superimposed on a screen 3.
The screen 3 distributes the projected light properly so that the projected light can be perceived as an image from various angles.
The screen 3 generally comprises a Fresnel lens sheet 4 and a lenticular lens array sheet 5. The Fresnel lens sheet 4 converges the projected light entering divergently from the center to the periphery of the screen and converts the projected light to a substantially parallel light. The lenticular lens array sheet 5 diffuses the projected light converted to the substantially parallel light so that the projected light can be perceived as an image from various angles.
The use of the lenticular lens array sheet as a means for diffusing the projected light instead of a simple diffusion sheet can implement the following effective functions.
The first function is providing anisotropic diffusion. The anisotropic diffusion makes it possible to effectively distribute a limited light and increase the luminance in the effective observation region. In an image display unit, it is generally required that bright good images be perceived from a wide angle range in the horizontal direction. On the other hand, good images should be perceivable in a range from the standing position to the sitting position in the vertical direction. It is said that when the effective observation region is expressed by an angle at which the luminance is reduced to a half of the front luminance (a half-luminance angle), the half-luminance angle should be about .+-.30.degree. in the horizontal direction and about .+-.10.degree. in the vertical direction. With isotropic diffusion, when the half-luminance angle in the horizontal direction is 30.degree., the half-luminance angle in the vertical direction is naturally 30.degree.. Therefore, the front luminance is 1/3compared with the case of the anisotropic diffusion.
The anisotropic diffusion is generally provided by adding a diffusion material inside the lenticular lens array sheet 5 so as to provide a relatively wide angle of view due to a synergetic effect of the action of the lenticular lens and the diffusion material in the horizontal direction and provide a relatively narrow angle of view due to the action of the diffusion material in the vertical direction to which the action of the lenticular lens does not contribute.
The second function is diffusing red, green and blue lights entering at different angles respectively with substantially the same light distribution characteristics. A phenomenon in which red, green and blue light ray groups have different directivities (light distribution characteristics) respectively due to different incidence angles is called color shift. Making the light distribution characteristics of each light ray group the same is called a color shift correction function. For the color shift correction function, a pair of lenticular lenses are provided on the entrance side and the exit side. The action of the lenticular lenses will be described with reference to FIG. 9.
FIG. 9 shows a cross-sectional view of an example of a pair of lenticular lens arrays that are designed to correct the color shift. The entrance surface and the exit surface are expressed by the following function wherein the x axis is the optical axis (the horizontal direction in FIG. 9) and the y axis is the direction perpendicular to the optical axis (the vertical direction in FIG. 9). ##EQU1##
In FIG. 9, for the light rays having light ray heights of 0 and .+-.0.53, the track of the light ray entering parallel to the optical axis (the green light ray) is indicated by the solid line, and the track of the light ray entering at 15.degree. with respect to the optical axis (the red or blue light ray) is indicated by the broken line.
As is apparent from FIG. 9, the lenticular lens 5b on the exit side corrects in such a manner that the light ray entering obliquely with respect to the optical axis exits at an angle substantially equal to that of the light ray entering parallel to the optical axis at the same light ray height. Thus, color change depending on the observation angle is prevented by making the diffusion profiles of the red and blue parallel light ray groups entering obliquely with respect to the optical axis substantially the same as the diffusion profile of the green light ray group entering parallel to the optical axis.
The third function is reducing the decrease of contrast due to the reflection of the outside light. As is apparent from FIG. 9, the light ray passage region on the exit surface is limited by the light gathering action of the lenticular lens 5a on the entrance side. Black stripes (light absorption layers) 6 are formed in the non-exit regions of the exit surface. A general method for forming the black stripes 6 comprises providing trapezoidal convex portions corresponding to the non-exit regions in forming the exit side lenticular lenses 5b and providing black stripes only in the convex portions by screen printing or transfer with a black ink, utilizing the unevenness. The black stripe 6 absorbs the outside light and reduces the decrease of contrast.
The reflection of the outside light without the black stripes is shown in FIG. 10. Without the black stripes, the light enters from the non-exit regions on the exit side as well. As a result, as shown in FIG. 10, 20 to 30% of the outside light entering from the exit surface is subjected to the total reflection of the surface of the entrance side lenticular lens 5a and exits to the observation side. With the black stripes, such a total reflection component can be blocked substantially completely.
Thus, the total reflection component on the surface of the entrance side lenticular lens can be reduced greatly by the black stripes. However, the unevenness is present on the exit surface because of the exit side lenticular lenses and the convex portions for forming the black stripes, and the unevenness causes the diffuse reflection of the outside light, thereby decreasing the contrast.
In order to reduce such diffuse reflection caused by the unevenness of the exit surface of the lenticular lens array sheet, a light transmission sheet containing a light absorption agent, that is, a tinted panel, is generally located on the observation side of the lenticular lens array sheet. If the tinted panel is present, the projected light passes through the tinted panel once (one passage), while the reflected component of the outside light reciprocates through the tinted panel (two passages). Therefore, the contrast can be relatively improved.
Furthermore, when a liquid crystal panel is used as the image source, the color shift correction function is unnecessary, and the exit side lenticular lenses are unnecessary. Therefore, the exit surface can be made flat to prevent the diffuse reflection due to the unevenness. However, when the liquid crystal panel is used as the image source, the reduction of the reflection of the lenticular lens array sheet is important in view of another factor.
When a liquid crystal panel is used as the image source, moire due to the interference of the periodic structure of the pixels and that of the lenticular lenses causes a problem. In order to avoid the moire problem, the pitch of the lenticular lenses should be sufficiently smaller than that of the pixels on the screen. Therefore, a lenticular lens array sheet having a finer pitch than the case of using CRTs as the image sources is required. It is difficult to form the black stripes in exact positions corresponding to the lenticular lenses as the pitch is finer.
Without the black stripes, a part of the outside light entering the lenticular lens array sheet is subjected to the total reflection due to the above-described mechanism, thereby deteriorating the contrast greatly.
In order to reduce such a contrast decrease due to the reflection of the surface of the entrance side lenticular lens, a light absorption agent is generally dispersed inside the lenticular lens array sheet.
As another method for reducing the effect on the outside light, a technology using "a blocking means" for transmitting light at a specific angle and blocking light at other angles is disclosed in Japanese Patent Application (Tokkai Hei) No. 7-056109. In this Japanese Patent Application, the above-described blocking means is located between the lenticular lens array sheet (simply referred to as "a screen" in the cited specification) on the observation side and the Fresnel lens sheet (referred to as "an aiming means" in the cited specification) on the projection side. According to such a means, the outside light entering the Fresnel lens sheet can be reduced significantly to prevent the contrast decrease caused by the reflection of the Fresnel lens sheet.
When the tinted panel is present or the light absorption agent is dispersed inside the lenticular lens array sheet as described above, the contrast certainly increases, but the loss of the projected light occurs. Furthermore, the efficiency for light utilization decreases greatly when trying to improve the contrast greatly.
The technology disclosed in Japanese Patent Application (Tokkai Hei) No. 7-056109 is effective for preventing the decrease of contrast caused by the entrance of the outside light into the Fresnel lens sheet and the reflection of the outside light inside the Fresnel lens sheet and the device. However, this technology does not have any effect of reducing the reflection of the outside light occurring in the lenticular lens array sheet as described above.